Adjuvant agents enhance round window membrane permeability to dexamethasone and modulate basal to apical cochlear gradients

Probucol-bile acid nanoparticles: a novel approach and promising solution to prevent cellular oxidative stress in sensorineural hearing loss

The use of antioxidants could thus prove an effective medication to prevent or facilitate recovery from oxidative stress-induced sensorineural hearing loss (SNHL). One promising strategy to prevent SNHL is developing probucol (PB)-based nanoparticles using encapsulation technology and administering them to the inner ear via the established intratympanic route. The preclinical, clinical and epidemiological studies support that PB is a proven antioxidant that could effectively prevent oxidative stress in different study models. Such findings suggest its applicability in preventing oxidative stress within the inner ear and its associated neural cells. However, several hurdles, such as overcoming the blood-labyrinth barrier, ensuring sustained release, minimising systemic side effects and optimising targeted delivery in the intricate inner ear structures, must be overcome to efficiently deliver PB to the inner ear. This review explores the background and pathogenesis of hearing loss, the potential of PB in treating oxidative stress and its cellular mechanisms, and the obstacles linked to inner ear drug delivery for effectively introducing PB to the inner ear.

Microbubble-assisted ultrasound for inner ear drug delivery

Treating pathologies of the inner ear is a major challenge. To date, a wide range of procedures exists for administering therapeutic agents to the inner ear, with varying degrees of success. The key is to deliver therapeutics in a way that is minimally invasive, effective, long-lasting, and without adverse effects on vestibular and cochlear function. Microbubble-assisted ultrasound ("sonoporation") is a promising new modality that can be adapted to the inner ear. Combining ultrasound technology with microbubbles in the middle ear can increase the permeability of the round window, enabling therapeutic agents to be delivered safely and effectively to the inner ear in a targeted manner. As such, sonoporation is a promising new approach to treat hearing loss and vertigo. This review summarizes all studies on the delivery of therapeutic molecules to the inner ear using sonoporation.

Proof of concept of intracochlear drug administration by laser-assisted bioprinting in mice

Transtympanic administration is used clinically for the injection of gentamicin and/or corticosteroids. This atraumatic route is based on passive diffusion through the round window membrane (RWM). The main limitation of this method is related to the clearance through the Eustachian tube, making the concentration of the therapeutic agent at the intracochlear level uncertain and limited. Moreover, this technique remains unsuitable for molecules of high molecular weight or in the case of gene therapies. The purpose was to study a new technique of intracochlear administration in an atraumatic, direct and controlled manner by laser-assisted bioprinting (LAB). LAB was used to deliver dexamethasone phosphate with thermosensitive hydrogel on the mouse RWM. After validation of the regularity and homogeneity of the pattern, the diffusion in vivo of the dexamethasone into the perilymph after LAB has been confirmed by ELISA. Auditory function measurements showed no hearing impairment suggesting that bioprinting does not induce significant cochlear damage. Hence, the present proof of concept study introduces a promising approach for inner ear drug delivery.

Safety and Efficacy of Intratympanic Histamine Injection as an Adjuvant to Dexamethasone in a Noise-induced Murine Model

The safety and efficacy of intratympanic (IT) histamine (HIS) injection as an adjuvant to increase the inner ear penetration of dexamethasone (DEX) was investigated in this study. IT injections of DEX-only, 1% HIS+DEX and 4% HIS+DEX were performed in mice with noise-induced hearing loss. An inflammatory reaction in the middle ear was observed only in the 4% HIS+DEX group although no serious cytotoxic effects on the organ of Corti (OC) were observed at that concentration. Compared with the DEX-only group, the perilymphatic concentration of DEX was approximately two times higher in the 1% HIS+DEX group and approximately five times higher in the 4% HIS+DEX group. The expression of the DEX receptor in the cochlea was significantly increased in the 4%-HIS+DEX group. HIS appeared to induce transient damage the microstructure of the RWM with recovery observed within 3 weeks. The 1% and 4% HIS + DEX groups showed a significant recovery of the OC compared with the control group and they also achieved significantly better hearing restoration at 8 kHz in the DPOAE hearing test (P < .05) when compared to the DEX-only group. IT HIS temporarily disrupts the structure of the RWM and middle ear mucosa and significantly enhances the inner ear penetration of DEX. Therefore, IT HIS injection could be a simple and effective adjuvant therapy to increase perilymph concentration of DEX and achieve OC recovery after cochlear damage.

Inner Ear Drug Delivery for Sensorineural Hearing Loss: Current Challenges and Opportunities

Most therapies for treating sensorineural hearing loss are challenged by the delivery across multiple tissue barriers to the hard-to-access anatomical location of the inner ear. In this review, we will provide a recent update on various pharmacotherapy, gene therapy, and cell therapy approaches used in clinical and preclinical studies for the treatment of sensorineural hearing loss and approaches taken to overcome the drug delivery barriers in the ear. Small-molecule drugs for pharmacotherapy can be delivered via systemic or local delivery, where the blood-labyrinth barrier hinders the former and tissue barriers including the tympanic membrane, the round window membrane, and/or the oval window hinder the latter. Meanwhile, gene and cell therapies often require targeted delivery to the cochlea, which is currently achieved via intra-cochlear or intra-labyrinthine injection. To improve the stability of the biomacromolecules during treatment, e.g., RNAs, DNAs, proteins, additional packing vehicles are often required. To address the diverse range of biological barriers involved in inner ear drug delivery, each class of therapy and the intended therapeutic cargoes will be discussed in this review, in the context of delivery routes commonly used, delivery vehicles if required (e.g., viral and non-viral nanocarriers), and other strategies to improve drug permeation and sustained release (e.g., hydrogel, nanocarriers, permeation enhancers, and microfluidic systems). Overall, this review aims to capture the important advancements and key steps in the development of inner ear therapies and delivery strategies over the past two decades for the treatment and prophylaxis of sensorineural hearing loss.

Injectable glycol chitosan thermogel formulation for efficient inner ear drug delivery

We prepared a new injectable thermogel to enhance the efficiency of inner ear delivery of dexamethasone (DEX). Hexanoyl glycol chitosan (HGC) was synthesized and evaluated as an amphiphilic thermogel (T_gel ~ 32 °C) for use as a solubilizing agent as well as an injectable carrier for intratympanic delivery of the hydrophilic and hydrophobic forms of DEX. Various thermogel formulations with different drug types and concentrations were prepared, and their physicochemical and thermogelling properties were characterized by ¹H NMR, ATR-FTIR, and rheometer. They exhibited versatile release kinetics from several hours to more than 2 weeks, depending on drug type and concentration. Our formulations further showed good residual stability for more than 21 days without any cytotoxicity or inflammation in the middle and inner ear and could deliver a considerably high drug concentration into the inner ear. Therefore, HGC thermogel has great potential as an effective and safe formulation for inner ear drug delivery.

Characterization of the Sheep Round Window Membrane

Intratympanic injection is a clinically used approach to locally deliver therapeutic molecules to the inner ear. Drug diffusion, at least in part, is presumed to occur through the round window membrane (RWM), one of the two openings to the inner ear. Previous studies in human temporal bones have identified a three-layered structure of the RWM with a thickness of 70-100 μm. This is considerably thicker than the RWM in rodents, which are mostly used to model RWM permeability and assess drug uptake. The sheep has been suggested as a large animal model for inner ear research given the similarities in structure and frequency range for hearing. Here, we report the structure of the sheep RWM. The RWM is anchored within the round window niche (average vertical diameter of 2.1 ± 0.3 mm and horizontal diameter of 2.3 ± 0.4 mm) and has a curvature that leans towards the scala tympani. The centre of the RWM is the thinnest (55-71 μm), with increasing thickness towards the edges (< 171 μm), where the RWM forms tight attachments to the surrounding bony niche. The layered RWM structure, including an outer epithelial layer, middle connective tissue and inner epithelial layer, was identified with cellular features such as wavy fibre bundles, melanocytes and blood vessels. An attached "meshwork structure" which extends over the cochlear aqueduct was seen, as in humans. The striking anatomical similarities between sheep and human RWM suggest that sheep may be evaluated as a more appropriate system to predict RWM permeability and drug delivery in humans than rodent models.

Impacts of different methylprednisolone administration routes in patients with sudden hearing loss or Meniere's disease

BACKGROUND

Evidence suggests that glucocorticoids are important in the treatment of sudden hearing loss (SHL) and Meniere's disease (MD). However, different glucocorticoid administration methods may have a significant impact on treatment outcomes.

OBJECTIVE

This study aimed to investigate effects of different glucocorticoid administration methods on sudden hearing loss and Meniere's disease.

METHODS

In this study, glucocorticoids were administered orally in 18 patients, by retroauricular injection in 15 patients and by intratympanic injection in 15 patients. White blood cell (WBC) count, serum K+, fasting plasma glucose (FPG), body temperature, heart rate and blood pressure were used to evaluate effects of glucocorticoids on patients with hearing loss. Visual analog scale (VAS) of pain and sleep disorders were also surveyed, and pure tone audiometry (PTA) results were compared among groups to evaluate efficacy of different glucocorticoids administration methods.

RESULT

WBC count, heart rate and blood pressure were higher in patients taking oral glucocorticoids, while body temperature, serum K+ and FPG levels did not change in all three groups. However, patients who received intratympanic injection of glucocorticoids experienced more pain, while those taking oral glucocorticoids reported more sleep impairment. Treatment efficacy on hearing loss was not significantly different among the three groups.

CONCLUSION

These findings suggest that systemic glucocorticoid administration can result in greater whole body responses than local administration, but with similar hearing treatment efficacy.

Dose-Dependent Transient Decrease of Impedances by Deep Intracochlear Injection of Triamcinolone With a Cochlear Catheter Prior to Cochlear Implantation-1 Year Data

Administration of low-dose steroids via a catheter inserted into the cochlea to apply pharmaceuticals to more apical regions was previously shown not to be sufficient for long-term reduction of electrode impedances. The aim of the present study was to investigate the effect of intra-cochlear high-dose triamcinolone application on impedances in cochlear implant recipients. Patients received low-dose (4 mg/ml; n = 5) or high-dose (20 mg/ml; n = 5) triamcinolone via a cochlear catheter just prior to the insertion of a Med-El Flex28 electrode. Impedances were measured at defined time points from intra-operatively up to 12 months after first fitting and retrospectively compared with a control group (no steroid application). Patients who received a high-dose application of crystalloid triamcinolone showed significantly reduced impedances in the first fitting measurements compared to the control group. This effect was no longer detectable in patients of the low-dose group at that time. Looking at the different regions of the electrode, the impedance values were lowered significantly only at the basal and medial contacts. At later time points, there were no significant differences between any of the groups. This is the first study to demonstrate a dose-dependent reduction of impedances by deep intra-cochlear injection of triamcinolone in cochlear implant patients. With a high-dose, single application of triamcinolone using a cochlear catheter prior to insertion of a Flex28 electrode, the impedances can be significantly reduced up to and including the first fitting. Although the effect was longer lasting than when compared to low-dose triamcinolone, it was also not permanent.

Partial Least Square Model (PLS) as a Tool to Predict the Diffusion of Steroids Across Artificial Membranes

One of the most challenging goals in modern pharmaceutical research is to develop models that can predict drugs’ behavior, particularly permeability in human tissues. Since the permeability is closely related to the molecular properties, numerous characteristics are necessary in order to develop a reliable predictive tool. The present study attempts to decode the permeability by correlating the apparent permeability coefficient (P_app) of 33 steroids with their properties (physicochemical and structural). The P_app of the molecules was determined by in vitro experiments and the results were plotted as Y variable on a Partial Least Squares (PLS) model, while 37 pharmacokinetic and structural properties were used as X descriptors. The developed model was subjected to internal validation and it tends to be robust with good predictive potential (R²Y = 0.902, RMSEE = 0.00265379, Q²Y = 0.722, RMSEP = 0.0077). Based on the results specific properties (logS, logP, logD, PSA and VD_ss) were proved to be more important than others in terms of drugs P_app. The models can be utilized to predict the permeability of a new candidate drug avoiding needless animal experiments, as well as time and material consuming experiments.

Ultrasound-induced microbubble cavitation via a transcanal or transcranial approach facilitates inner ear drug delivery

Ultrasound-induced microbubble (USMB) cavitation is widely used to promote drug delivery. Our previous study investigated USMB targeting the round window membrane by applying the ultrasound transducer to the tympanic bulla. In the present study, we further extended the use of this technology to enhance drug delivery to the inner ear by introducing the ultrasound transducer into the external auditory canal (EAC) or applying it to the skull. Using a 3-dimensional-printed diffusion apparatus mimicking the pathway for ultrasound passing through and reaching the middle ear cavity in vitro, the models simulating the transcanal and transcranial approach demonstrated 4.8-fold- and 3.7-fold-higher delivery efficiencies, respectively. In an in vivo model of guinea pigs, by filling tympanic bulla with microbubbles and biotin-FITC, USMB applied transcanally and transcranially induced 2.8-fold and 1.5-fold increases in biotin-FITC delivery efficiencies, respectively. In addition, the gentamicin uptake by cochlear and vestibular hair cells and gentamicin-induced hair cell loss were significantly enhanced following transcanal application of USMB. On the 28th day after transcanal USMB, safety assessment showed no significant changes in the hearing thresholds and the integrity of cochlea. These are the first results to our knowledge to demonstrate the feasibility and support the potential clinical application of applying USMB via EAC to facilitate drug delivery into the inner ear.

Inner ear delivery: Challenges and opportunities

OBJECTIVES

The treatment of inner ear disorders remains challenging due to anatomic barriers intrinsic to the bony labyrinth. The purpose of this review is to highlight recent advances and strategies for overcoming these barriers and to discuss promising future avenues for investigation.

DATA SOURCES

The databases used were PubMed, EMBASE, and Web of Science.

RESULTS

Although some studies aimed to improve systemic delivery using nanoparticle systems, the majority enhanced local delivery using hydrogels, nanoparticles, and microneedles. Developments in direct intracochlear delivery include intracochlear injection and intracochlear implants.

CONCLUSIONS

In the absence of a systemic drug that targets only the inner ear, the best alternative is local delivery that harnesses a combination of new strategies to overcome anatomic barriers. The combination of microneedle technology with hydrogel and nanoparticle delivery is a promising area for future investigation.

LEVEL OF EVIDENCE

NA.

Ultrastructural Changes Associated With the Enhanced Permeability of the Round Window Membrane Mediated by Ultrasound Microbubbles

The round window membrane (RWM) is the most common entryway for local drug and gene delivery into the inner ear, but its permeability can change the treatment outcome. We previously demonstrated a feasible and highly efficient approach using ultrasound-aided microbubble (USMB) cavitation to enhance the permeability of the RWM. Here, we investigated the safety of USMB exposure and the association between temporal changes in RWM permeability and ultrastructure. Experimental guinea pigs were divided into two treatment groups: a control group receiving round window soaking (RWS) with MBs and treatment (USM) groups undergoing 3 (USM-3) or 5 (USM-5) consecutive USMB exposures (1 min/exposure) at an acoustic intensity of 3 W/cm² and 1 MHz frequency. The trans-RWM delivery efficiency of biotin-fluorescein isothiocyanate conjugates, used as permeability tracers, revealed a greater than 7-fold higher delivery efficiency for the USM groups immediately after 3 or 5 exposures than for the RWS group. After 24 h, the delivery efficiency was 2.4-fold higher for the USM-3 group but was 6.6-fold higher for the USM-5 group (and 3.7-fold higher after 48 h), when compared to the RWS group. Scanning electron microscopy images of the RWM ultrastructure revealed USMB-induced sonoporation effects that could include the formation of heterogeneous pore-like openings with perforation diameters from 100 nm to several micrometers, disruption of the continuity of the outer epithelial surface layer, and loss of microvilli. These ultrastructural features were associated with differential permeability changes that depended on the USMB exposure course. Fourteen days after treatment, the pore-like openings had significantly decreased in number and the epithelial defects were healed either by cell expansion or by repair by newly migrated epithelial cells. The auditory brainstem response recordings of the animals following the 5-exposure USMB treatment indicated no deterioration in the hearing thresholds at a 2-month follow-up and no significant hair cell damage or apoptosis, based on scanning electron microscopy, surface preparations, and TUNEL assays. USMBs therefore appear to be safe and effective for inner ear drug delivery. The mechanism of enhanced permeability may involve a disruption of the continuity of the outer RWM epithelial layer, which controls transmembrane transport of various substances.

Local Drug Delivery for Prevention of Hearing Loss

Systemic delivery of therapeutics for targeting the cochlea to prevent or treat hearing loss is challenging. Systemic drugs have to cross the blood-labyrinth barrier (BLB). BLB can significantly prevent effective penetration of drugs in appropriate concentrations to protect against hearing loss caused by inflammation, ototoxic drugs, or acoustic trauma. This obstacle may be obviated by local administration of protective agents. This route can deliver higher concentration of drug compared to systemic application and preclude systemic side effects. Protective agents have been administered by intra-tympanic injection in numerous preclinical studies. Drugs such as steroids, etanercept, D and L-methionine, pifithrin-alpha, adenosine agonists, melatonin, kenpaullone (a cyclin-dependent kinase 2 (CDK2) inhibitor) have been reported to show efficacy against cisplatin ototoxicity in animal models. Several siRNAs have been shown to ameliorate cisplatin ototoxicity when administered by intra-tympanic injection. The application of corticosteroids and a number of other drugs with adjuvants appears to enhance efficacy. Administration of siRNAs to knock down AMPK kinase, liver kinase B1 (LKB1) or G9a in the cochlea have been found to ameliorate noise-induced hearing loss. The local administration of these compounds appears to be effective in protecting the cochlea against damage from cisplatin or noise trauma. Furthermore the intra-tympanic route yields maximum protection in the basal turn of the cochlea which is most vulnerable to cisplatin ototoxicity and noise trauma. There appears to be very little transfer of these agents to the systemic circulation. This would avoid potential side effects including interference with anti-tumor efficacy of cisplatin. Nanotechnology offers strategies to effectively deliver protective agents to the cochlea. This review summarizes the pharmacology of local drug delivery by intra-tympanic injection to prevent hearing loss caused by cisplatin and noise exposure in animals. Future refinements in local protective agents provide exciting prospects for amelioration of hearing loss resulting from cisplatin or noise exposure.